[CI] Introduce a separate child pipeline for unit tests; allow executing on multiple PHP images

Problem/Motivation

After commit of #3477529: [CI] Remove the 'with-database' requirement for unit tests, we do not need to run Unit tests for each db configuration anymore.

Steps to reproduce

Proposed resolution

A separate child pipeline, accommodating also PHPStan rule unit tests.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet